Re: [RFC PATCH v1] mm: improve call_controls_lock

[Gutierrez Asier <gutierrez.asier@huawei-partners.com>]

On 12/31/2025 10:51 AM, JaeJoon Jung wrote:
> On Wed, 31 Dec 2025 at 15:10, JaeJoon Jung <rgbi3307@gmail.com> wrote:
>>
>> On Wed, 31 Dec 2025 at 13:59, SeongJae Park <sj@kernel.org> wrote:
>>>
>>> On Wed, 31 Dec 2025 11:15:00 +0900 JaeJoon Jung <rgbi3307@gmail.com> wrote:
>>>
>>>> On Tue, 30 Dec 2025 at 00:23, SeongJae Park <sj@kernel.org> wrote:
>>>>>
>>>>> Hello Asier,
>>>>>
>>>>>
>>>>> Thank you for sending this patch!
>>>>>
>>>>> On Mon, 29 Dec 2025 14:55:32 +0000 Asier Gutierrez <gutierrez.asier@huawei-partners.com> wrote:
>>>>>
>>>>>> This is a minor patch set for a call_controls_lock synchronization improvement.
>>>>>
>>>>> Please break description lines to not exceed 75 characters per line.
>>>>>
>>>>>>
>>>>>> Spinlocks are faster than mutexes, even when the mutex takes the fast
>>>>>> path. Hence, this patch replaces the mutex call_controls_lock with a spinlock.
>>>>>
>>>>> But call_controls_lock is not being used on performance critical part.
>>>>> Actually, most of DAMON code is not performance critical.  I really appreciate
>>>>> your patch, but I have to say I don't think this change is really needed now.
>>>>> Please let me know if I'm missing something.
>>>>
>>>> Paradoxically, when it comes to locking, spin_lock is better than
>>>> mutex_lock
>>>> because "most of DAMON code is not performance critical."
>>>>
>>>> DAMON code only accesses the ctx belonging to kdamond itself. For
>>>> example:
>>>> kdamond.0 --> ctx.0
>>>> kdamond.1 --> ctx.1
>>>> kdamond.2 --> ctx.2
>>>> kdamond.# --> ctx.#
>>>>
>>>> There is no cross-approach as shown below:
>>>> kdamond.0 --> ctx.1
>>>> kdamond.1 --> ctx.2
>>>> kdamond.2 --> ctx.0
>>>>
>>>> Only the data belonging to kdamond needs to be resolved for concurrent access.
>>>> most DAMON code needs to lock/unlock briefly when add/del linked
>>>> lists,
>>>> so spin_lock is effective.
>>>
>>> I don't disagree this.  Both spinlock and mutex effectively work for DAMON's
>>> locking usages.
>>>
>>>> If you handle it with a mutex, it becomes
>>>> more
>>>> complicated because the rescheduling occurs as a context switch occurs
>>>> inside the kernel.
>>>
>>> Can you please elaborate what kind of complexities you are saying about?
>>> Adding some examples would be nice.
>>>
>>>> Moreover, since the call_controls_lock that is
>>>> currently
>>>> being raised as a problem only occurs in two places, the kdamon_call()
>>>> loop
>>>> and the damon_call() function, it is effective to handle it with a
>>>> spin_lock
>>>> as shown below.
>>>>
>>>> @@ -1502,14 +1501,15 @@ int damon_call(struct damon_ctx *ctx, struct
>>>> damon_call_control *control)
>>>>         control->canceled = false;
>>>>         INIT_LIST_HEAD(&control->list);
>>>>
>>>> -       mutex_lock(&ctx->call_controls_lock);
>>>> +       spin_lock(&ctx->call_controls_lock);
>>>> +       /* damon_is_running */
>>>>         if (ctx->kdamond) {
>>>>                 list_add_tail(&control->list, &ctx->call_controls);
>>>>         } else {
>>>> -               mutex_unlock(&ctx->call_controls_lock);
>>>> +               spin_unlock(&ctx->call_controls_lock);
>>>>                 return -EINVAL;
>>>>         }
>>>> -       mutex_unlock(&ctx->call_controls_lock);
>>>> +       spin_unlock(&ctx->call_controls_lock);
>>>>
>>>>         if (control->repeat)
>>>>                 return 0;
>>>
>>> Are you saying the above diff can fix the damon_call() use-after-free bug [1]?
>>> Can you please elaborate why you think so?
>>>
>>> [1] https://lore.kernel.org/20251231012315.75835-1-sj@kernel.org
>>>
>>
>> The above code works fine with spin_lock.  However, when booting the kernel,
>> the spin_lock call trace from damon_call() is output as follows:
>> If you have any experience with the following, please share it.
>>
>> [    0.834450] Call Trace:
>> [    0.834456] [<ffffffff8001b376>] dump_backtrace+0x1c/0x24
>> [    0.834471] [<ffffffff800024e0>] show_stack+0x28/0x34
>> [    0.834480] [<ffffffff80014f4c>] dump_stack_lvl+0x48/0x66
>> [    0.834493] [<ffffffff80014f7e>] dump_stack+0x14/0x1c
>> [    0.834503] [<ffffffff800032c6>] spin_dump+0x62/0x6e
>> [    0.834511] [<ffffffff80087376>] do_raw_spin_lock+0xd0/0x128
>> [    0.834523] [<ffffffff80de9378>] _raw_spin_lock+0x1a/0x22
>> [    0.834538] [<ffffffff80255c0c>] damon_call+0x38/0x100
>> [    0.834548] [<ffffffff8025f022>] damon_stat_start+0x10e/0x168
>> [    0.834558] [<ffffffff80e21ab4>] damon_stat_init+0x2a/0x44
>> [    0.834568] [<ffffffff800157c0>] do_one_initcall+0x40/0x202
>> [    0.834579] [<ffffffff80e015f6>] kernel_init_freeable+0x1fc/0x27e
>> [    0.834588] [<ffffffff80de0a9e>] kernel_init+0x1e/0x13c
>> [    0.834599] [<ffffffff8001716a>] ret_from_fork_kernel+0x10/0xf8
>> [    0.834607] [<ffffffff80deab22>] ret_from_fork_kernel_asm+0x16/0x18
>> [    0.943407] NFS: Registering the id_resolver key type
>> [    0.948996] Key type id_resolver registered
>> [    0.953614] Key type id_legacy registered
> 
> The above occurred because spin_lock_init() was not performed.  The problem
> is that spin_lock_init() was not added while deleting mutex_init().
> Please refer to the contents below.
> 
> @@ -539,6 +539,7 @@ struct damon_ctx *damon_new_ctx(void)
> 
>         mutex_init(&ctx->kdamond_lock);
>         INIT_LIST_HEAD(&ctx->call_controls);
> -       mutex_init(&ctx->call_controls_lock);
> +       spin_lock_init(&ctx->call_controls_lock);

Right, my bad.

I can submit a new updated patch with this small change. However,
I believe, as discussed before, that the improvement in general will 
be small and it may not be worth it.

>         mutex_init(&ctx->walk_control_lock);
> 
>         ctx->attrs.min_nr_regions = 10;
> 
>>
>> Thanks,
>> JaeJoon
>>
>>>
>>> Thanks,
>>> SJ
>>>
>>> [...]
> 

-- 
Asier Gutierrez
Huawei